Contrasted Liquid Lines of Descent Revealed by Olivine-hosted Melt Inclusions and the External Magma

Abstract : To better understand the significance of the chemical compositions of forsterite-hosted melt inclusions, dynamic crystallization experiments were performed at atmospheric pressure and a low cooling rate (2°C h−1) on a starting glass material with a composition in the CMAS system (12·21 wt % CaO, 28·48 wt % MgO, 11·96 wt % Al2O3, 47·32 wt % SiO2). Experiments were quenched at various temperatures, both above and below the theoretical solidus. Olivine is the liquidus phase and the mesostasis may exhibit a second crystalline phase corresponding to a metastable Al-rich pyroxene. Olivine crystal morphologies change from polyhedral to skeletal, depending on the quenching temperature. Regardless of the quenching temperature, the chemical compositions of liquids trapped in melt (now glass) inclusions in the olivines do not contain the boundary layer that should result from rapid growth of these crystals. The liquid compositions also show that down to temperatures far below the theoretical solidus, olivine is the only phase that crystallizes on the walls of these inclusions. Glass inclusion analyses can therefore be used to determine the chemical evolution of a liquid in metastable equilibrium with the host olivine, irrespective of the quenching temperature. Melt inclusions therefore follow their own liquid line of descent distinct from that of the whole charge. This experimental study suggests that it should be possible to (1) draw the metastable extensions of the liquidus from natural glass inclusions and (2) determine the real supersaturation degrees of the residual magmas.
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F. Faure, L. Tissandier. Contrasted Liquid Lines of Descent Revealed by Olivine-hosted Melt Inclusions and the External Magma. Journal of Petrology, Oxford University Press (OUP), 2014, 55 (9), pp.1779 - 1798. ⟨10.1093/petrology/egu041⟩. ⟨hal-01765242⟩

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